Process for the production of liquid hydrocarbons

ABSTRACT

A process for producing normally liquid hydrocarbon products from a hydrocarbonaceous feedstock, especially from normally gaseous hydrocarbon feed, having the following steps: (a) partial oxidation of the normally gaseous hydrocarbon feed at elevated pressure using air or oxygen enriched air as oxidant, to obtain a syngas mixture having hydrogen, carbon monoxide and nitrogen; (b) converting hydrogen and carbon monoxide obtained in step (a) into a normally liquid hydrocarbon product and a normally gaseous hydrocarbon product; (c) separating from the reaction mixture obtained in step (b) an off-gas mixture having nitrogen, normally gaseous hydrocarbon product, and unconverted hydrogen, carbon monoxide and normally gaseous hydrocarbon feed, insofar as such unconverted components are present; (d) combusting at least a part of the off-gas mixture in a steam raising apparatus, producing steam of an elevated pressure; and (e) expanding the steam produced in step (d) for compressing the air or oxygen enriched air and/or the normally gaseous hydrocarbon feed used in step (a); and a plant having equipment in a line-up suitable for carrying out the process.

FIELD OF THE INVENTION

The present invention relates to a process for the production of liquidhydrocarbons from a gaseous hydrocarbon feed, especially theoptimisation of an integrated, low-cost process for the production ofnormally liquid hydrocarbons from natural gas or especially associatedgas, at remote locations or at offshore locations.

BACKGROUND OF THE INVENTION

Many publications (cf. for example WO-94/21512, WO-97/12118, WO-91/15446and U.S. Pat. No. 4,833,170) describe processes for the conversion of(gaseous) hydrocarbon feed, such as methane, natural gas and/orassociated gas, into liquid products, especially methanol and liquidhydrocarbons, particularly paraffinic hydrocarbons. Such conversionprocesses may be operated at remote locations (e.g. in desserts,tropical rain-forests) and/or offshore locations, where no direct use ofthe gas is possible, due to the absence of large populations andindustries. Transportation of the gas to populated and industrial areas,e.g. through a pipeline or in the form of liquefied natural gas,requires extremely high capital expenditure or is simply not practical.This holds even more in the case of relatively small gas productionfields and/or relatively small gas production rates. Re-injection of gasinto the production field will add to the costs of the oil production,and may, in the case of associated gas, result in undesired effects onthe crude oil production. Burning of associated gas has become undesiredin view of depletion of hydrocarbon sources and air pollution.

Gas found together with crude oil is known as associated gas, whereasgas found separate from crude oil is known as non-associated gas.Associated gas may be found as “solution gas” dissolved within the crudeoil, and/or as “gas cap gas” adjacent to the main layer of crude oil.Associated gas is usually much richer in the larger hydrocarbonmolecules (ethane, propane, butane) than non-associated gas.

Especially in view of the fact that the above-mentioned conversionprocesses may be operated at remote locations or at locations wherelimited space is available there is an incentive to place specialemphasis on such factors as energy and cost efficiency, compactness andcomplexity of the process or the plant in which the process is carriedout. From the references given above, however, no optimally integrated,efficient, low-cost process scheme is available.

WO-98/01514 discloses a process in which gaseous hydrocarbon feed isconverted with air into syngas which, in turn, is converted into liquidhydrocarbon product in a Fischer-Tropsch synthesis step. A substantialamount of the heat generated in the process is recovered and re-used inthe process. Further, an off-gas mixture which is co-produced in theFischer-Tropsch synthesis is used to fuel a gas turbine which, in turn,is used to power the compressor needed for compressing the air used inthe process. The off-gas mixture in question comprises unconvertedsyngas, methane by-product from the Fischer-Tropsch synthesis, andnitrogen originating from the air used. The use of air as the oxidant inthe conversion of the gaseous hydrocarbon obviates the need of aproduction unit of an oxygen rich oxidant. However, the nitrogen presentin the air acts in the process as a diluent gas, necessitating handlinglarger quantities of gas at a higher total pressure, which requires morecompression capacity.

A further disadvantageous aspect of the use of air is that the saidoff-gas mixture is diluted with nitrogen which causes that it has a lowheating value. The heating value is especially low when the syngasproduction and the Fischer-Tropsch synthesis are operated efficiently,so that the content of combustible materials in the off-gas mixture isfurther decreased. In the light of the process of WO-98/01514 this willrepresent a problem when the heating value is so low that the off-gasmixture is unsuitable for use as a gas turbine fuel.

SUMMARY OF THE INVENTION

It has now been found that when the off-gas mixture is unsuitable foruse as a gas turbine fuel, sufficient energy for operating thecompressors, and even for operating the whole process, can be recoveredfrom the off-gas mixture by burning the off-gas mixture for theproduction of steam and using the steam as the source of shaft powerand/or electrical power. This finding leads to an integrated, highlyefficient, low-cost process with low capital and space requirements forthe production of normally liquid hydrocarbons from normally gaseoushydrocarbons. Further, there is-no need for the importation ofadditional fuel or other sources of energy for operating the process.The process has a high carbon efficiency, which means that a highproportion of the carbon present in the hydrocarbon feed is present inthe normally liquid hydrocarbon products.

DETAILED DESCRIPTION OF THE INVENTION

The present finding may be applied especially when associated gas is thefeedstock, which, after separation from the crude oil, is usuallyavailable at low pressure or even at ambient pressure only. The presentfinding may also be applied when the feed is gas from low pressure gasfields or largely depleted gas fields, having only a low remainingpressure. In a preferred embodiment, the process may be carried out in acompact, relatively light weight plant, making it very suitable for useon a platform or a barge, or in a dismountable plant.

A major advantage of the present finding is that relatively simple andcheap processes and apparatus can be used. Further, an optimal use offeedstock and energy is obtained. In a preferred embodiment an optimumcarbon conversion (gas into syncrude, minimal carbon dioxide emission),is obtained. In addition, the normally liquid hydrocarbons produced maybe mixed with crude oil and transported together.

The present invention thus provides a process for producing normallyliquid hydrocarbon products from a hydrocarbonaceous feedstock,especially from a normally gaseous hydrocarbon feed, which comprises thefollowing steps:

(a) partial oxidation of the normally gaseous hydrocarbon feed atelevated pressure using air or oxygen enriched air as oxidant, to obtaina syngas mixture comprising hydrogen, carbon monoxide and nitrogen;

(b) converting hydrogen and carbon monoxide obtained in step (a) into anormally liquid hydrocarbon product and a normally gaseous hydrocarbonproduct;

(c) separating from the reaction mixture obtained in step (b) an off-gasmixture comprising nitrogen, normally gaseous hydrocarbon product, andunconverted hydrogen, carbon monoxide and normally gaseous hydrocarbonfeed, insofar as such unconverted components are present;

(d) combusting at least a part of the off-gas mixture in a steam raisingapparatus, producing steam of an elevated pressure; and

(e) expanding the steam produced in step (d) for compressing the air oroxygen enriched air and/or the normally gaseous hydrocarbon feed used instep (a).

The invention also relates to a plant comprising equipment in a line-upsuitable for carrying out the process of this invention.

The normally gaseous hydrocarbon feed is suitably methane, natural gas,associated gas or a mixture of C₁₋₄ hydrocarbons, preferably associatedgas. The C₁₋₄ hydrocarbons or mixtures thereof are gaseous attemperatures between about 5 to about 30° C. at 1 bara (i.e. barabsolute), especially at about 20° C. at 1 bara. The normally gaseoushydrocarbon feed comprises mainly, i.e. more than about 80% v,especially more than about 90% v, C₁₋₄ hydrocarbons. The normallygaseous hydrocarbon feed comprises especially at least about 60% vmethane, preferably at least about 75% v, more preferably at least about90% v. Very suitably natural gas or associated gas is used, especiallyassociated gas at a remote location or at an offshore location. In somecases the natural gas or the associated gas comprises in addition carbondioxide and/or nitrogen, e.g. in amounts up to about 15% v or even up toabout 25% v of each of these compounds on the normally gaseoushydrocarbon feed.

The normally liquid hydrocarbons mentioned in the present descriptionare suitably C₄₋₂₄ hydrocarbons, especially C₅₋₂₀ hydrocarbons, moreespecially C₆₋₁₆ hydrocarbons, or mixtures thereof. These hydrocarbonsor mixtures thereof axe liquid at temperatures between about 5 to about30° C. at 1 bara, especially at about 20° C. at 1 bara, and usually areparaffinic of nature, although considerable amounts of olefins and/oroxygenates may be present. The normally liquid hydrocarbons may compriseup to about 20% w, preferably up to about 10% w, of either olefins oroxygenated compounds. Depending on the catalyst and the processconditions used, also normally solid hydrocarbons may be obtained. Thesenormally solid hydrocarbons may be formed in the Fischer-Tropschreaction in amounts up to about 85% w based on total hydrocarbonsformed, usually between about 50 to about 75% w.

The normally gaseous hydrocarbon product comprises mainly, i.e. morethan about 80% v, especially more than about 90% v, C₁₋₄ hydrocarbons.These hydrocarbons or mixtures thereof are gaseous at temperaturesbetween about 5 to about 30° C. at 1 bara (i.e. bar absolute),especially at about 20° C. at 1 bara, and usually are paraffinic ofnature, although considerable amounts of olefins and/or oxygenates maybe present. The normally gaseous hydrocarbon product comprisesespecially at least about 30% v methane, preferably at least about 40%v, more preferably at least about 50% v. The normally gaseoushydrocarbon product may comprise up to about 20% w, preferably up toabout 10% w, of either olefins or oxygenated compounds.

Suitably, any sulphur in the normally gaseous hydrocarbon feed isremoved, for example, in an absorption tower a sulphur binding agent,such as iron oxide or zinc oxide.

The partial oxidation of the normally gaseous hydrocarbon feed,producing the syngas mixture can take place according to variousestablished processes. These processes include the Shell GasificationProcess. A comprehensive survey of this process can be found in the Oiland Gas Journal, Sep. 6, 1971, pp. 86-90. The reaction is suitablycarried out at a temperature between 800 and 2000° C. and a pressurebetween 4 and 80 bara.

Oxygen for use in step (a) is sourced from air or from oxygen enrichedair. The oxygen enriched air gas comprises suitably up to about 70% voxygen, preferably up to about 60% v, in particular in the range of fromabout 25 to about 40% v. Oxygen enriched air may be produced bycryogenic techniques, but preferably it is produced by a process whichis based on separation by means of a membrane, such as disclosed inWO-93/06041. The use of oxygen enriched air is not a preferred option.Preferably air is employed in step (a).

Very suitable processes for partial oxidation are catalytic partialoxidation processes, especially as described in EP-A-576096,EP-A-629578, EP-A-645344, EP-A-656317 and EP-A-773906. In the catalyticpartial oxidation processes a catalyst bed may be applied. Suitablestructures of the catalyst bed are monolith structures, especiallyceramic foams, but also metal based structures may be used. Themonolithic structures may comprise inorganic materials of hightemperature resistance, selected from compounds of elements of GroupsIIa, IIIa, IVa, IIIb, IVb and the lanthanide group of the Periodic Tableof the Elements. Preferably the monolithic structure is zirconia based,especially stabilised zirconia. Suitable active metals for the catalyticpartial oxidation process are rhodium, platinum, palladium, osmium,iridium and ruthenium, and mixtures thereof. Preferably, rhodium and/oriridium is used.

The temperature applied in the catalytic partial oxidation is usuallybetween about 700 to about 1300° C., suitably between about 800 to about1200° C., preferably between about 850 to about 1050° C., and thepressure is usually between about 4 to about 80 bara, suitably betweenabout 10 to about 50 bara, preferably between about 15 to about 40 bara.

The GHSV is suitably in the range of from about 50,000 to about100,000,000 Nl/l/h, preferably from about 500,000 to about 50,000,000Nl/l/h, especially from about 1,000,000 to about 20,000,000 Nl/l/h. Theterm “GHSV” is well known in the art, and relates to the gas per hourspace velocity, i.e. the volume of synthesis gas in Nl (i.e. at thestandard temperature of 0° C. and the standard pressure of 1 bara(100,000 Pa)) which is contacted in one hour with one litre of catalystparticles, i.e. excluding inter-particular void spaces. In the case of afixed bed catalyst, the GHSV is usually expressed as per litre ofcatalyst bed, i.e. including inter-particular void space. In that case aGHSV of about 1.6 Nl/l/h on catalyst particles corresponds frequentlywith about 1.0 Nl/l/h on catalyst bed.

To adjust the H₂/CO ratio of the syngas mixture, carbon dioxide and/orsteam may be introduced into the partial oxidation process. As asuitable steam source, water which is co-produced in step (b) may beused. As a suitable carbon dioxide source, carbon dioxide from theeffluent gasses of the combustion of step (d) may be used. The H₂/COratio of the syngas mixture is suitably between about 1.5 to about 2.3,preferably between about 1.8 to about 2.1.

If desired, a small amount of hydrogen may be made separately, forexample, by steam reforming of gaseous normally hydrocarbon feed,preferably in combination with the water shift reaction, and added tothe syngas mixture. Any carbon monoxide and carbon dioxide producedtogether with the hydrogen may be used as additional feed in step (b),or it may be recycled to step (a) to increase the carbon efficiency.Alternatively, it may be combusted in step (d), together with or inadmixture with the normally gaseous hydrocarbon product.

To keep the process as simple as possible, separate hydrogen manufacturewill usually not be a preferred option. Likewise, it is not a preferredoption to remove any nitrogen from the syngas mixture, or from any othernormally gaseous product mixture described in this patent document.

In another embodiment the H₂/CO ratio of the syngas mixture may bedecreased by removal of hydrogen from the syngas mixture. This can bedone by conventional techniques, such as pressure swing adsorption orcryogenic processes. A preferred option is a separation based onmembrane technology. In the case that hydrogen is removed from thesyngas mixture it may be preferred to apply a two-stage conversion instep (b). The hydrogen is then mixed with the gaseous products of thefirst stage, and together introduced in the second stage. The C₅+selectivity (i.e. the selectivity to hydrocarbons containing 5 or morecarbon atoms, expressed as a weight percentage of the total hydrocarbonproduct) can be improved in this line-up. A portion of the hydrogen maybe used in an optional, additional hydrocracking step in whichespecially the heavier fraction of the hydrocarbons produced in step (b)is cracked, as set out hereinafter.

Typically the normally gaseous hydrocarbon feed fed to the partialoxidation of step (a) is completely converted therein. Frequently, thepercentage of hydrocarbon feed which is converted amounts to 50-99% wand more frequently 80-98% w, in particular 85-96% w.

It is preferred that the heat generated in the partial oxidation isrecovered for re-use in the process. For example, the syngas mixtureobtained in step (a) may be cooled, typically to a temperature betweenabout 100 to about 500° C., suitably between about 150 to about 450° C.,preferably between about 200 to about 400° C. Preferably, the cooling iseffected in a steam raising apparatus, such as a boiler, withsimultaneous generation of steam typically of an elevated pressure.Further cooling to temperatures between about 30 to about 130° C.,preferably between about 40 to about 100° C., may be accomplished in aconventional heat exchanger, especially in a tubular heat exchanger forexample against cooling water or against the feed led to the reactor, orin an air cooler against air.

To remove any impurities from the syngas mixture, a guard bed may beused. Especially to remove all traces of HCN and/or NH₃ specific typesof active coal may be used. Trace amounts of sulphur may be removed byan absorption process using iron oxide and/or zinc oxide.

In step (b) the syngas mixture is converted into the normally liquidhydrocarbons and normally gaseous hydrocarbons. Suitably at least about70% v of the syngas (i.e. the portion of the syngas mixture consistingof hydrogen and carbon monoxide) fed to step (b), is converted.Preferably all the syngas fed to step (b) is converted, but frequentlyabout 80 to about 99% v, more frequently about 90 to about 98% v isconverted. Typically all of the syngas obtained in step (a) is fed intostep (b) and more typically all of the syngas mixture obtained in step(a) is fed into step (b).

The conversion of step (b) of hydrogen and carbon monoxide intohydrocarbons is well known in the art and it is herein referred to bythe usual term “Fischer-Tropsch synthesis”. Catalysts for use in theFischer-Tropsch synthesis frequently comprise, as the catalyticallyactive component, a metal from Group VIII of the Periodic Table ofElements. Particular catalytically active metals include ruthenium,iron, cobalt and nickel. Cobalt is a preferred catalytically activemetal. Typically, at least a part of the catalytically active metal ispresent in metallic form.

The catalytically active metal is preferably supported on a porouscarrier. The porous carrier may be selected from any of the refractorymetal oxides or silicates or combinations thereof known in the art.Particular examples of preferred porous carriers include silica,alumina, titania, zirconia, ceria, gallia and mixtures thereof,especially silica and titania.

The amount of catalytically active metal present in the catalyst ispreferably in the range of from about 3 to about 75% w, more preferablyfrom about 10 to about 50% w, especially from about 15 to about 40% w,relative to the weight of the catalyst.

If desired, the catalyst may also comprise one or more metals or metaloxides as promoters. Suitable metal oxide promoters may be oxides ofelements selected from Groups IIA, IIIB, IVB, VB and VIB of the PeriodicTable of Elements, and the actinides and lanthanides. In particular,oxides of magnesium, calcium, strontium, bariwn, scandium, yttrium,lanthanum, cerium, titanium, zirconium, haihium, thorium, uranium,vanadium, chromium and manganese are most suitable promoters.Particularly preferred metal oxide promoters for the catalyst used inthe present invention are manganese and zirconium oxide. Suitable metalpromoters may be selected from Groups VIIB and VIII of the PeriodicTable. Rhenium and Group VIII noble metals are particularly suitable,with platinum and palladium being especially preferred. The amount ofpromoter present in the catalyst is suitably in the range of from about0.01 to about 50% w, preferably about 0.1 to about 30% w, morepreferably about I to about 15% w, relative to the weight of thecatalyst.

The catalytically active metal and the promoter, if present, may bedeposited on the carrier material by any suitable treatment, such asimpregnation, kneading and extrusion. After deposition of the metal and,if appropriate, the promoter on the carrier material, the loaded carrieris typically subjected to calcination at a temperature generally in therange of from about 350 to about 750° C., preferably in the range offrom about 450 to about 550° C. After calcination, the resultingcatalyst may be activated by contacting the catalyst with hydrogen or ahydrogen-containing gas, typically at a temperature in the range of fromabout 200 to about 350° C. Particular forms of catalyst are shellcatalysts, in which the catalytically active metal and the promoter, ifpresent, are positioned in the outer layer of relatively coarse catalystparticles, e.g. extrudates (cf. e.g. U.S. Pat. No. 5,545,674 and thereferences cited therein), and catalysts which are present in the formof a powder, e.g. a spray dried powder, suitable for forming a slurry inthe liquid reaction medium of the Fischer-Tropsch synthesis (cf. e.g.WO-99/349 17).

Preferably a catalyst is used which comprises cobalt on a titaniacarrier, because such a catalyst is highly efficient in theFischer-Tropsch synthesis in that it provides a high conversion ofsyngas combined with a high C₅+ selectivity, when compared with othercatalysts, thus a low production of the gaseous hydrocarbon products.Preferably, the catalyst contains a further metal selected frommanganese, vanadium, zirconium, rhenium, scandium, platinum andruthenium. Preferably the further metal is manganese or vanadium, inparticular manganese.

The Fischer-Tropsch synthesis may conveniently and advantageously beoperated in a single pass mode (“once through”) devoid of any recyclestreams, thus allowing the process to be comparatively simple andrelatively low cost. The process may be carried out in one or morereactors, either parallel or in series. In the case of small hydrocarbonfeedstock streams, the preference will be to use only one reactor.Slurry bubble reactors, ebulliating bubble reactors and fixed bedreactors may be used. In order to minimise the production of gaseoushydrocarbon product, it is preferred to apply fixed bed reactor incombination with a shell type catalyst, or a reactor in combination witha powdery catalyst which is present as a slurry in the liquid reactionmedium.

The Fischer-Tropsch synthesis may be performed under conventionalconditions known in the art. Typically, the temperature is in the rangeof from about 100 to about 450° C., preferably from about 150 to about350° C., more preferably from about 180 to about 270° C. Typically, thetotal pressures is in the range of from about 1 to about 200 bara, morepreferably from about 20 to about 100 bara. The GHSV may be chosenwithin wide ranges and is typically in the range from about 400 to about10000 Nl/l/h, for example from about 400 to about 4000 Nl/l/h.

It is preferred that the heat generated in the Fischer-Tropsch synthesisis recovered for re-use in the process. For example, the Fischer-Tropschreaction mixture may be cooled with simultaneous generation of steamtypically of an elevated pressure. This may be done outside the reactorin which the Fischer-Tropsch synthesis is carried out, for example in aconventional heat exchanger, or inside the reactor, for example byemploying a multi-tubular reactor or by means of an internal coolingcoil. Typically the Fischer-Tropsch reaction mixture may finally becooled to a temperature between about 40 to about 130° C., preferablybetween about 50 to about 100° C., by means of a conventional heatexchanger, especially in a tubular heat exchanger for example againstcooling water or against the feed led to the reactor, or in an aircooler against air.

The product of step (b), i.e. the product of the Fischer-Tropschsynthesis, is separated in step (c) into the off-gas mixture and afraction comprising the normally liquid hydrocarbon products and,suitably, a fraction comprising the water which is co-produced in theFischer-Tropsch synthesis. This separation may involve distillation andphase separation and it may be carried out using conventional equipment,for example a distillation column or a gas/liquid separator andoptionally a liquid/liquid separator. The off-gas mixture comprisesnitrogen, the normally gaseous hydrocarbon product, and unconvertedhydrogen, carbon monoxide and normally gaseous hydrocarbon feed, if anyof such unconverted components is present. Besides nitrogen, the off-gasmixture may comprise further non-combustible components such as carbondioxide and inert gasses such as helium.

The pressure of the off-gas mixture is substantially the same as thepressure prevailing in the Fischer-Tropsch synthesis reactor used instep (b). If the pressure is above 1 bara, it is advantageous to expandthe off-gas mixture, preferably in a turbine using its mechanical energyfor compression purposes. It is preferred to use the mechanical energyof the off-gas mixture for compression of the syngas mixture prior tobeing fed to step (b). This effects that the Fischer-Tropsch synthesisof step (b) is performed at a relatively high pressure which leads to abetter efficiency of the Fischer-Tropsch synthesis, in particular ahigher conversion rate, while the partial oxidation of step (a) isperformed at a relatively low pressure, at which the partial oxidationis more efficient, in terms of a better conversion level. Preferably thepressure increase of the syngas mixture amounts to at least about 5 bar,in particular about 10 to about 50 bar, more in particular about 15 toabout 40 bar.

In step (d), at least a part of the off-gas mixture, preferably at leastabout 90% w, in particular all of the off-gas mixture, is combusted in asteam raising apparatus, generating steam of an elevated pressure.Preferably the off-gas mixture as fed to the steam raising apparatus isslightly above ambient pressure, typically in the range of about 1.01 toabout 5 bara, more typically about 2 to about 4 bara.

The steam raising apparatus may be conventional equipment, such as afurnace equipped with heating coils, a boiler or a superheater. Thepressure of the steam generated may be at least about 2 bara.Preferably, steam of various pressures is generated simultaneously, forexample, a low pressure steam, a medium pressure steam and a highpressure steam. The low pressure steam has a pressure of in the range ofabout 2 to about 8 bara, preferably in the range of about 3 to about 5bara. The medium pressure steam has a pressure of in the range of about8 to about 40 bara, preferably in the range of about 10 to about 30bara. The high pressure steam has a pressure of in the range of about 40to about 100 bara, preferably in the range of about 50 to about 80 bara.Preferably, for its efficient use in step (e), the steam produced issuperheated steam. Typically the degree of superheating is at leastabout 5° C. For practical reasons the degree of superheating is at mostabout 100° C. Typically the degree of superheating is in the range offrom about 20 to about 80° C.

Customarily, the heating value of a gas is expressed quantitatively byits “lower heating value”. For easy combustion of the off-gas mixture inthe conventional steam raising apparatus, it is preferred that theoff-gas mixture has a composition such that its lower heating value isin the range of from about 3 to about 15 MJ/Nm³ (“Nm³” refers to the gasvolume at 0° C., 1 bara). Preferably, the lower heating value of theoff-gas is in the range of about 3.5 to about 11 MJ/Nm³, more preferablyin the range of about 4 to about 6 MJ/Nm³. The lower heating value ofthe off-gas mixture can be determined experimentally or, if thecomposition of the off-gas mixture is known, it can be calculated byadding up the weighted contributions of the lower heating value of theindividual components. The lower heating value of the relevant compoundsare known to the skilled person.

In a preferred embodiment of the process the heat of combustionrecovered in step (d) is used together with the heat of reactionrecovered in step (a) and/or the heat of reaction recovered in step (b)for producing steam. For example, superheated high pressure steam havinga pressure in the range of about 60 to about 65 bara may be produced byheating water to form steam using the heat of reaction recovered in step(a), followed by superheating the steam using the heat of combustion ofthe off-gas mixture. Alternatively, or preferably simultaneously,superheated medium pressure steam having a pressure in the range ofabout 15 to about 25 bara may be produced by heating water using theheat of combustion of the off-gas mixture, which steam is then combinedwith steam of equal pressure produced in the Fischer-Tropsch synthesisreactor of step (b), and subsequently superheated using the heat ofcombustion of the off-gas mixture. The various steps which involveheating using the heat of combustion of the off-gas mixture may be donesimultaneously in a single furnace in which the off-gas mixture iscombusted, by using a plurality of heating coils.

In accordance with this invention, at least a part of the steam producedis used for compressing the air or oxygen enriched air and/or thenormally gaseous hydrocarbon feed. In first embodiment the steam isemployed as a source of shaft power by using for the compression, acompressor which is driven by a steam turbine. For example, highpressure steam of about 60 to about 65 bara, preferably superheatedsteam of that pressure, may be employed in the steam turbine forcompressing the air or oxygen enriched air used in step (a).Alternatively, or preferably simultaneously, medium pressure steam ofabout 15 to about 25 bara, preferably superheated steam of thatpressure, may be employed in another steam turbine which drives acompressor compressing the normally gaseous hydrocarbon feed used instep (a). In a second, less preferred embodiment the steam is employedas a source of electrical power by using for the compression acompressor which is driven electrically and the electrical power neededis generated using the power of a steam turbine which is driven be steamgenerated in step(e).

A further quantity of steam may be used for generating power which isused elsewhere in the process. In particular, it may be used forgenerating electricity, which is used for driving any electricalequipment used in the process, other than the electrically drivencompressors mentioned hereinbefore (if any), such as pumps, air blowers,and other. Sometimes there is a surplus of energy, which may be appliedoutside the process.

The normally liquid hydrocarbon product as obtained from the step (b)may be transported in liquid form or mixed with any stream of crude oilwithout creating problems as to solidification and or crystallisation ofthe mixture. It is observed in this respect that in step (b) heavyhydrocarbons products, such as C₁₈₋₂₀₀ hydrocarbons, in particularC₂₀₋₁₀₀ hydrocarbons, may be coproduced which show a tendency tosolidify as waxy materials, in which case the normally liquidhydrocarbon product becomes more difficult in its handling.

If this is the case, but also for other reasons, at least part of thehydrocarbon product may be subjected to a catalytic hydrocracking, whichis known per se in the art. The catalytic hydrocracking is carried outby contacting the normally liquid hydrocarbon product at elevatedtemperature and pressure and in the presence of hydrogen with a catalystcontaining one or more metals having hydrogenation activity, andsupported on a carrier. Suitable hydrocracking catalysts includecatalysts comprising metals selected from Groups VIB and VIII of thePeriodic Table of Elements. Preferably, the hydrocracking catalystscontain one or more noble metals from group VIII. Preferred noble metalsare platinum, palladium, rhodium, ruthenium, iridium and osmium. Mostpreferred catalysts for use in the hydrocracking stage are thosecomprising platinum. To keep the process as simple as possible, theapplication of a additional catalytic hydrocracking step will usuallynot be a preferred option.

The amount of catalytically active metal present in the hydrocrackingcatalyst may vary within wide limits and is typically in the range offrom about 0.05 to about 5% w, relative to the weight of the catalyst.Suitable conditions for the catalytic hydrocracking are known in theart. Typically, the hydrocracking is effected at a temperature in therange of from about 175 to about 400° C. Typical hydrogen partialpressures applied in the hydrocracking process are in the range of fromabout 10 to about 250 bara.

The catalytic hydrocracking may be carried out before the separation ofstep (c), but preferably it is carried out after the separation of step(c). Additional normally gaseous hydrocarbon products (i.e. hydrocarbonproducts or a mixture thereof which are gaseous at temperatures betweenabout 5 to about 30° C. at 1 bara, especially at about 20° C. at about 1bara) may be formed during the catalytic hydrocracking. Any off-gas ofthe catalytic hydrocracking, comprising any unconverted hydrogen and theadditional normally gaseous hydrocarbon product formed, may be separatedfrom the catalytic hydrocracking reaction product, and added to and/orcombusted with the off-gas mixture in step (d).

It is an advantage of this invention that the process can be carried outwithout the need of having available gas which has a high heating valuefor fueling a gas turbine for power generation, so that it canadvantageously be used as feedstock in the process for the conversioninto normally liquid hydrocarbon product. Thus, the normally gaseoushydrocarbon feed can be used completely for conversion purposes.Further, the off-gas mixture may be of a low heating value, which meansthat the partial oxidation and the Fischer-Tropsch synthesis may beoperated at a high efficiency so that the process is performed with ahigh carbon efficiency.

There is no need to import energy for operating the process from sourcesoutside the process and/or to install a gas turbine for powergeneration. In the case that energy would be imported from a sourceoutside the process, for example for a reason of convenience, thequantity of energy imported will be less than about 50%, preferably lessthan about 25%, more preferably less than about 10% relative to theenergy needed for operating the process, i.e. the total energy needed todrive the energy consuming equipment employed in the process, such asheat generating equipment, compressors, pumps, air blowers, and other.

The hydrocarbonaceous feed is preferably a normally gaseous hydrocarbonfeed, but may also be a solid hydrocarbon feed, e.g. coal, brown coal,peat or organic waste.

The process may be carried out at a remote location and/or offshore, forexample on a vessel or platform.

What is claimed is:
 1. A process for producing normally liquidhydrocarbon products from a hydrocarbonaceous feedstock, which processcomprises the following steps: (a) partially oxidating a normallygaseous hydrocarbon feed at elevated pressure using air or oxygenenriched air as oxidant, to obtain a syngas mixture comprising hydrogen,carbon monoxide and nitrogen; (b) converting hydrogen and carbonmonoxide obtained in step (a) into a normally liquid hydrocarbon productand a normally gaseous hydrocarbon product; (c) separating from thereaction mixture obtained in step (b) an off, gas mixture comprisingnitrogen, normally gaseous hydrocarbon product, and unconvertedhydrogen, carbon monoxide and normally gaseous hydrocarbon feed; (d)combusting at least a part of the off-gas mixture in a steam raisingapparatus, producing steam of an elevated pressure; and (e) expandingthe steam produced in step (d) for compressing the air or oxygenenriched air and/or the normally gaseous hydrocarbon feed used in step(a).
 2. The process of claim 1, wherein the hydrocarbon feed is selectedfrom the group consisting of methane, natural gas, associated gas and amixture of C₁₋₄ hydrocarbons.
 3. The process of claim 1, wherein air isused as the oxidant.
 4. The process of claim 1, wherein the partialoxidation is a catalytic partial oxidation carried out at a temperaturebetween about 800° C. to about 1200° C. and a pressure between about 10bara to about 50 bara.
 5. The process of claim 1, wherein the conversionof hydrogen and carbon monoxide is carried out at a temperature in therange of from about 150° C. to about 350° C., a total pressure in therange of from about 1 bara to about 200 bara, a GHSV in the range fromabout 400 Nl/l/h to about 10000 Nl/l/h and using a catalyst whichcomprises cobalt on a titania carrier.
 6. The process of claim 1,wherein the off-gas mixture has a lower heating value in the range ofabout 3.5 MJ/Nm³ to about 11 MJ/Nm³.
 7. The process of claim 1, furthercomprising: recovering the heat of reaction produced in step (a) and/orin step (b) and the heat of combustion produced in step (d) is usedtogether with the heat of reaction recovered in step (a) and/or the heatof reaction recovered in step (b) for producing steam.
 8. The process ofclaim 1, wherein the energy used for operating the process which isimported from sources outside the process, is less than about 25%relative to the energy needed for operating the process.